P23B9NOx Sensor Heater Sense Circuit High (Bank 1 Sensor 3)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- P23B9 is the counterpart of P23B8: the temperature feedback wire of NOx sensor 3 on bank 1 carries a voltage that only a short to battery or to another live wire can explain. The sensor then cannot report a trustworthy heater temperature, so it withholds its NOx output. GM applies this check to the post-second-catalyst sensor in 25OBDG04B (2025) under the same frame as the ground short: powertrain relay above 11 V, sensor supply above 9.90 V, dew point reached, and half of 160 samples (80) must fail before this Type B code is stored. A harness overheated near the exhaust, where insulation melts between neighbouring wires, is a classic trigger; the same 21-NA-149 bulletin for the 3.0L LM2 includes P23B9. P2211 and P22A9 cover sensors 1 and 2.
- What you may notice
- Check engine light after two trips
- No NOx reading from sensor 3
- Possibly a blown fuse in the sensor supply
- Likely causes
- LikelySense wire shorted to the heater supply or another battery-fed wire
- PossibleHarness insulation melted by contact with hot exhaust parts
- PossibleInternal fault in the sensor or its control unit
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €20 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.